The present invention relates generally to hot swap controllers which can be inserted into and/or removed from a power supply system, such as the power supply system of a server back plane, to connect or disconnect power to a particular load while power continues to be supplied to the load by means of other hot swap controllers which are already plugged into the power supply system. More particularly, the invention relates to a hot swap controller that is capable of providing minimum voltage loss, minimum power dissipation, and maximum power utilization efficiency.
The closest prior art is believed to include the brochure “hp Carrier Grade Blade Server bh3710 Site Preparation Guide, Edition 2.0”, copyright 2002-2004 by Hewlett-Packard Development Company, L. P., the publication “Passive Current Sharing Boosts Power and Reliability” by Berry Ehrman, Power Electronics Technology, pages 58 and 56, January, 2005, and the published Application Note 13, Rev B, by RO Associates, Inc. “Paralleling with Current Sharing and N+M Redundancy”, pages 13-1 through 13-4, Oct. 5, 1995.
Prior Art FIG. 1 shows a power supply 1 which operates by means of passive load current sharing. A main DC power supply voltage VDC is applied to the power inputs of two regulated power supplies 2 and 3, designated Power Supply #1 and Power Supply #2, respectively. Power supplies #2 and #3 may be conventional DC-DC converters having VDC as a common input reference voltage (which typically might be 12 volts), or they may be DC switching power supplies on the back plane of a server rack.
The −Vout terminals of power supplies 2 and 3 are connected to a LOAD RETURN conductor 7. The +Vout terminal of power supply 2 is connected to the anode of a Schottky diode D1, the cathode of which is connected by conductor 4 to one terminal of a ballast resistor R1. The other terminal of ballast resistor R1 is connected by conductor 5 to a load (not shown). Similarly, the +Vout terminal of power supply 3 is connected to the anode of another Schottky diode D2, the cathode of which is connected by conductor 6 to one terminal of another ballast resistor R2. The other terminal of ballast resistor R2 is connected to load conductor 5. A load circuit (not shown) is connected between LOAD conductor 5 and LOAD RETURN conductor 7. The “passive load sharing” illustrated in FIG. 1 is accomplished by means of the circuitry including diodes D1 and D2 and resistors R1 and R2. For more information, see the above mentioned article by Ehrman. Power supply 1 typically is included on a PC board that also includes a particular load circuit, wherein the PC board would be inserted into the back plane, e.g., server back plane, of an equipment rack. Power supply 1 of FIG. 1 can be considered to be general prior art for passive current sharing. (It is believed that none of the known prior art current sharing is specific to hot swap controllers.)
In the past, ballast resistors such as R1 and R2 have been used in various current sharing circuits to accomplish approximately equal sharing of current from two or more power supplies (or other power-supplying elements such as power transistors). Each ballast resistor has one terminal coupled to the load and another terminal coupled to a power supply, respectively. The ballast resistor technique has been an effective way of providing relatively equal sharing of current from the two or more power-supplying elements, but suffers from the major disadvantage that a substantial amount of power is dissipated in the ballast resistors and therefore is wasted.
FIG. 2 shows a prior art n+1 redundant power supply system 10 which utilizes n the ballast resistors to achieve relatively equal sharing of load current from multiple power supplies or other current-supplying elements. In FIG. 2, an input power supply voltage or reference voltage VDC is applied to a power terminal of a typical server back plane 9 to which n power-supplying elements are connected. The main back plane supply voltage VDC is applied between back plane power bus conductors 11 and 12. Conductor 11 is coupled by fuses F1, F2 . . . Fn and conductors 13-1,2 . . . n to the +IN inputs of n power supplies 20-1, 20-2 . . . 20-n, respectively, which are illustrated as MICROVERTER™ DC-DC converters. Conductor 12 is connected to the −IN input of power supplies 20-1, 2 . . . n. A PDM 15-1 has a VCC terminal coupled by resistor R1 to conductor 13-1 and a ground terminal connected to conductor 12. (The term “PDM” is an abbreviation for “Paralleling De-Coupling Module”, which is a trademark of RO Associates, author of the above mentioned published application note.) Similarly, a PDM 15-2 has a VCC terminal coupled by another resistor R1 to conductor 13-2 and a ground terminal connected to conductor 12, and a PDM 15-n has a VCC terminal coupled by another resistor R1 to conductor 13-n and a ground terminal connected to conductor 12. A “SHARE” terminal of each PDM is connected to the “SHARE” terminals of the other two. (The SHARE terminal has a function unique to the RO Associates PDM.). A bypass capacitor C4 is connected between the +IN and −IN terminals of each of power supplies 20-1, 2 . . . n, respectively. A “PAR” terminal of each of PDMs 15-1, 2 . . . n is connected to a corresponding PAR terminal of each of power supplies 20-1, 2 . . . n, respectively. (The PAR terminals are RO Associates terminology for the pins to be connected to each other to enable parallel operation of their DC-DC converters.)
For each of the power supplies 20-1, 2 . . . n, a “+SENSE” terminal is connected to a conductor 21, which is connected to one terminal of a load 25. A “−SENSE” terminal is connected to one terminal of a resistor R5 and one terminal of a resistor R6. The other terminal of resistor R5 is connected by conductor 22 to the other terminal of load 25. The other terminal of resistor R6 is connected to a +OUT terminal of the power supply and to the anode of a diode D2, the anode of which is connected to load 25 by means of conductor 22. This circuit does not use ballast resistors but rather depends more on the diodes D2, which also suffer excessive power losses. (For further information on the system of Prior Art FIG. 2, see the above mentioned publication “Paralleling With Current Sharing And N+M Redundancy”, RO Associates, Inc., Application Note 13, Rev B, Oct. 5, 1995.
Unfortunately, the n+1 redundant power supply system 10 of Prior Art FIG. 2 has the shortcoming of high power dissipation loss which is unacceptable in state-of-the-art hot swap controller applications. The paralleling circuitry is complex and not readily adaptable in any form to hot swap controllers.
Hot swap controllers in blade servers typically are 40 ampere systems that connect a shunt resistor and power switching MOSFET (metal oxide semiconductor field effect transistor) in series with a 12 volt supply that comes from the back plane of a server rack to feed a server plugged into the rack. Voltage drops and associated wasted power dissipation in this path must be held to a minimum. Industry standards at the present time specify less than 30 millivolt drops across the power MOSFET, and existing hot swap current limiting circuitry causes additional drops of up to nearly 50 millivolts, for total a voltage drop of less than 80 millivolts at a 40 ampere level of load current. It would be desirable for future hot swap controller systems to reduce the voltage drop across the current-sensing shunt to 12 millivolts from the present 50 millivolt standard, lowering the total voltage drop to less than 42 millivolts. It should be noted that the total impedance of existing systems is on the order of 2 milliohms, and it is expected that the total output impedance will be reduced to roughly 1 milliohm in the not-too-distant future.
It should be appreciated that hot swap controllers and associated power MOSFETs usually operate in an environment in which only very low voltage drops can be tolerated, in order to prevent wasted power dissipation. It is very difficult in such an environment to connect multiple hot swap controllers in parallel such that they approximately equally share the total load current so as to avoid damage to edge connector pins of the circuit cards. In a typical server application (and any other typical high-current application), the amount of current being drawn may be, for example, roughly 40 amperes, and a FET (field effect transistor) that is used as a switch would normally be desired to have a source-to-drain voltage drop of roughly 30 millivolts with a drain current of about 40 amperes in a 12 volt DC system. That 30 millivolt source-drain voltage drop is typical of the presently acceptable amount of voltage loss in such state-of-the-art high current systems. Most of the prior art current sharing methods that have been used involve voltage drops of the order of 100 millivolts across all of the elements involved in accomplishing the current sharing, such as resistors, diodes, or combinations thereof. But in light of the present state-of-the-art system design and energy conservation considerations, that 100 millivolts voltage drop is roughly triple what would be desirable in the relatively near future.
A hot swap controller can be connected to and disconnected from the main power supply voltage in a back plane while the power is on. It is highly desirable that this be achieved without disrupting the regulated voltage being delivered to various circuitry included in the load that is being supplied by the back plane supply voltage. When a circuit card is inserted into a rack (e.g., into a server back plane) which continues to supply power to a number of other plugged-in circuit cards by means of various hot swap controllers, the power supply bypass capacitors usually provided on the circuit cards being inserted present a very low initial impedance that tends to momentarily short-circuit the back plane supply voltage. The resulting momentary decrease in the regulated supply voltage provided to the circuitry of the various already-plugged-in circuit cards may cause malfunctions in one or more of the already-plugged-in circuit cards. That could cause serious data errors, for example in a server application.
The term “pin” herein refers to the two edge connector leads or the like of the circuit card that conducts the “hot” +Vout voltage of each of power supplies 2 and 3 to load conductor 5 in order to prevent too much of the load current from flowing through a single edge connector lead. The term “pin” also refers to the two edge connector leads of the circuit card that conducts the −Vout voltage of each of power supplies 2 and 3 to load return conductor 7 in order to prevent too much of a load return current from flowing through a single edge connector lead.
One function of a hot swap controller is to limit the flow of current into the above mentioned bypass capacitors on the circuit board/card being plugged into the back plane to a sufficiently low level that the back plane voltage will not drop below its specified minimum value. If the current into the bypass capacitors is not sufficiently limited, the card edge pin conducting the unlimited current and/or the corresponding conductor of the socket into which the pin is plugged may be burned or damaged, causing a reliability problem.
The hot swap controller circuit also controls the ramping rate of its own output voltage. When its proper output voltage has been attained, the hot swap controller turns its output transistor (which typically is a switching MOSFET) completely on, and also provides a signal to the remainder of whatever system the hot swap controller is part of (which can be one of many types of systems, including server systems) to indicate that the back plane supply voltage is adequate for reliable operation of the various circuit cards.
Again, it must be emphasized that it is essential that the wasted power be minimized in server back plane and hot swap controller systems. It would be highly desirable to provide a hot swap controller in which the total current being supplied to the load is equally supplied by the two or more power supplies (or other current-supplying elements) without incurring nearly as much loss or waste of power as occurs in the ballast resistors of the closest prior art hot swap controllers.
For example, if the 12 volt pins in a typical prior art hot swap controller have a 42 millivolt difference at full current, that 42 millivolt difference is sufficient to cause all of the current to be “hogged” by the higher voltage pin, likely resulting in damage to that pin and the socket into which it is plugged. (The “hogging” occurs because the resistance of copper traces is of a low enough magnitude that currents become very high at even low voltages, in this case distributing all of the available current into a path designed to handle only half of the unavailable current.)
It would be desirable for future servers to receive the 12 volt DC supply voltage via a pair of edge connector pins that are connected to the back plane, each server being supplied by its own hot swap controller. However, a problem in achieving this goal arises when edge connector pins have a mismatch in voltage. For example, a 42 millivolt difference between the two pins means that a single pin would carry essentially the entire current. It would be desirable to have a means to ensure that the total load current is equally shared between two or more connector pins.
The most common and useful prior methods of current sharing have simply introduced sufficient ballast resistance into the multiple feeds paths of a circuit to reduce mismatch in the output voltage of each power supply, etc. to acceptable levels. For example, use of source ballast resistors or emitter ballast resistors is common in transistor circuits to enhance the needed current sharing and prevent current hogging. However, all of these prior art load current sharing methods incur excessive losses in this system when they use values of resistance that accomplish an acceptable amount of current sharing.
There is an unmet need for a controller circuit which can provide approximately equal load current sharing among multiple power-supplying elements without using ballast resistors.
There also is an unmet need for a hot swap controller which can achieve approximately equal load current sharing and can also provides substantially reduced power dissipation compared to hot swap controllers of the prior art.
There also is an unmet need for a hot swap controller which can achieve approximately equal load current sharing without using ballast resistors.
There also is an unmet need for a hot swap controller which substantially reduces current hogging among parallel-coupled hot swap controllers and at the same time substantially reduces the amount of wasted power dissipation compared to the amount of power wasted in prior parallel-coupled hot swap controllers.